Storage layer including arsenic, selenium and at least one of zinc, silver, cadmium and mercury



May 3, 1966 R. A. SIMMS ETAL 3,249,782

STORAGE LAYER INCLUDING ARSENIC, SELENIUM AND AT LEAST ONE OF ZINC, SILVER, CADMIUM AND MERCURY Filed Feb. 1, 1963 2 Sheets-Sheet 1 Fig.5.

' RELATIVE SPECTRAL RESPONSE PERCENT RESPONSE I I I I 3000 4000 5000 6000 7000 8000 WAVELENGTH ANGSTROMS WITNESSES INVENTORS Robert A. Simms 8| Albert J. Cornish R. A. SIMMS ETAL 9,782 G ARSENIC, SELENIUM AND AT LEAST ONE OF ZINC, SILVER, CADMIUM AND MERCURY 2 Sheets-Sheet 2 LlGHT TRANSFER CHARACTERISTICS AT STATED TARGET VOLTAGES May 3, 1966 STORAGE LAYER INCLUDIN Filed Feb. 1, 1963 2 |l S U 4. 3 -4 O .m m 6 v F F m :6 S w E I L 8 W n5 D N 4 L NE 0 l AC GR V CR A .U 4 1 O C l0 1 4 ws mA E 4 R FN SA M n H H N C mm m Q TN I AU m NT 2 W 4 0 12 U7 L Lu T I. R -l A m .e R l I O m RE 6 2 8 O P EM 3 3 2 M 8 mmmmlE OmO S F2mmmDU ZQ w H 2265 amk mwmkz 2323212 "-0 kzmommn.

TIME-MINUTES the surface of the antimony trisulfide.

United States Patent Filed Feb. 1, 1963, Ser. No. 255,606 11 Claims. (Cl. 313-65) This invention relates to a storage device and more particularly to a target member for said storage device and a method of manufacturing said target member.

A storage device is used in those applications wherein it is desired to store signals of variable intensities and then read the stored inforamtion out at a later time. The device normally in an evacuated envelope consists of a dielectric storage target member upon which is established a pattern of charges representative of the information to be stored. The charge pattern is normally established by directing information to be stored in the form of a beam or electromagnetic radiation such as visible light onto the storage target member.

In a storage tube of the type described above, it is desirable in many applications to perform the operation of reading out the charge image or'pattern Without removing the charge image or pattern and thus permit reading out of the information as many times as desired. This may be referred to as a multicopy type of storage tube. It is also desirable that such a tube provides half tone information, that is, signals between the maximum and minimum signals, applied to the tube in all copies.

At the present time at least three types of storage devices, namely, the metrechon, the transmission type storage display tube and a storage tube as described in U.S. Patent 3,046,431, issued July 24, 1962, entitled Storage System, by I. F. Nicholson and assigned to the same assignee as this invention provide multicopy readout with half tone. This invention is particularly directed to an improved target structure which exhibits the effects and may be incorporated in a storage system such as described in the above-mentioned patent.

. In the storage device described in the above-mentioned patent, a photoconductive pickup storage tube embodiment is described. The device consists essentially of a target formed with a light transmissive electrically conductive coating supported on a face plate portion of the envelope and a layer of arsenic and selenium deposited on the conductive coating with a layer of antimony trisulfide deposited on the arsenic and selenium layer. This target structure is mounted within an evacuated envelope and the exposed surface of the antimony trisulfide layer faces an electron gun structure. The electron gun struc ture generates a low velocity type beam and provides a pencil-like reading electron beam substantially normal to The electrons Within the reading beam approach the target with very low energies, normally below the first cross over potential of the target surface. These electrons are deposited on the photoconductive surface to drive the surface to substantially the cathode potential of the reading electron beam gun. When this potential, normally referred to as an equilibrium potential, is reached, the remaining electrons in the electron beam are reflected back toward the electron gun.

There is also applied to the electrically conductive signal back plate of the target a potential of about volts positive with respect to the equilibrium potential on the photoconductive surface. In this manner, a potential gradient is established across the two layers of material. Due to the photoconductive properties of the formation on the target member.

- potential.

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target materials used, light directed onto photoconductive materials will cause a charge to be established at the scan surface of the target. This change in charge or potential of the surface will be toward the potential of the electrically conductive back plate. The electron beam scan ning over the target areas illuminated by light will derive a signal from the target in an' output circuit connected to the signal plate but will not destructively readout the in- In fact, it is found that it is necessary to keep the scanning beam turned on to retain the stored information on the target member. The scene viewed by the pickup tube can be cut-01f after a short exposure or the exposure to the scene can be continued. By continued exposure, it is found that the signal on the target will integrate and build up a larger signal. If desired, the pattern written on the target can be read out for a considerable length of time In addition to light stimulation or writing as indicated above or by a device such as described in a copending application Serial #56572, filed September 16, 1960, now Patent No. 3,124,715, entitled Storage Device by G. Cox and assigned to same assignee as this invention, information may be written onto the target structure by an electron beam scanning the exposed surface of the photoconductive layer. The writing electron beam may be positioned on the opposite side of the target withirespect to the reading beam and by use of electrons of adequate energy to penetrate the electrically conductive layer can induce conductivity within the arsenic and selenium layer and the antimony trisulfide layer by electron bombardment induced conductivity. An electron bombardment induced conductivity type of device is described in U.S. Patent No. 2,900,555, assigned to the same assignee as this invention. In addition, U.S. application Serial No. 855,887, filed November 27, 1959, now U.S. Patent No. 3,148,297, entitled, Storage System, by J. Nicholson et a1. and assigned to the same assignee as the present invention describes this type of assembly.

The length of time for providing multicopy readout may be modified by modifying the amount of potential across the target-such as varying the signal back plate In addition, the stored image on the target may be removed by exposing the target to high level light illumination or by turning the scan read beam off for a period of time of about one minute.

Experience with the device described in U.S. Patent 3,046,431 has indicated the desirabilities of providing a single layer target of increased readout storage time and sensitivity.

It is, therefore, an object of this invention to provide an improved radiation sensitive storage electrode.

It is another object to provide an improved storage electrode.

It is another object to provide an improved target for use in an electron bombardment induced conductivity type tube.

It is another object to provide an improved storage device having a long period of continuous readout of information stored on a storage member.

In accordance with our invention, we provide an improved storage target electrode by evaporating an admixture of arsenic and selenium with the addition or substitution of certain materials including zinc, cadmium, mercury, silver, sulphur, phosphorous, antimony and tellurium.

Further objects and advantages of the invention will become apparent as the following description proceeds. The features of novelty which characterize the invention will be pointed out in particularity in claims annexed to and forming a part of the description.

For a better understanding of the invention, reference may be had to the accompanying drawings, in which:

FIGURE 1 is a view in section of a pickup tube embodying the teachings of this invention;

FIGURE 2 is an enlarged sectional view of the target schematically shown in FIGURE 1;

FIGURE 3 is a graphical representation of the integrated spectral sensitivity of the target in FIGURE 1 with the maximum video signal obtained over the integration period at a given wavelength;

FIGURE 4 is a graphical representation illustrating the signal current derived from the target in FIGURE 1 in response to a given illumination; and

FIGURE 5 is a graphical representation of percent of maximum integrated signal versus time to illustrate the properties of the target in FIGURE 1.

Referring now to FIGURE 1, a pickup storage tube is illustrated. The envelope, electron gun, scanning and deflection system are similar to those used in the conventional vidicon type of pickup tube. The vidicon is a well-known television pickup tube for producing a video signal for transmission.

The tube shown in FIGURE 1 comprises an envelope of a suitable material such as glass. One end of the envelope 10 is closed by an end wall or face plate portion 12 through which electromagnetic waves such as visible light from the scene viewed enter and energize a target member 14. The face plate 12 is also of a suitable material transmissive to the radiation from scene being viewed. Glass is a suitable material for viewing scenes of visible light. The interior surface of the face plate window 12 is provided with an electrically conductive coating 16 which is also transmissive to the radiations from the scene. A suitable material for the coating 16 is stannic oxide. The input screen or storage member 14 consists of a layer of an evaporated admixture of a suitable material such as CdAs Se S deposited on the electrically conductive coating 16. The electrically conductive coating 16 is connected to the exterior of the envelope by a lead 60. The lea-d 60 is connected to one terminal of a resistor 62 with the other terminal of the resistor 62 connected to a voltage source 64. The voltage source 64 supplies a potential to the coating 16 which may be within the range of about 2 to 50 volts.

The other end of the tube envelope 10 provides the base of a tube to which various leads (not shown) enter into the tube for applying suitable potentials to the electrodes therein. The tube is provided with the necessary wellknown components needed to produce and control a pencil-like beam of electrons to scan across the target 14 in a point-by-point manner. An electron gun 17 is provided within the tube adjacent the base to generate and form the electron beam. The electron gun 17 may be of any suitable design and includes a cathode 20, a control grid 28, screen grid 30 and an accelerating electrode 32. The cathode 20 is comprised of a tubular sleeve 22 closed at one end facing the screen or target 14. The closed end of the sleeve 22 is provided with a suitable thermionic emission coating to provide electron emission. A heater 26 is provided within the sleeve 22 to heat the electron emissive coating. The electrons emitted from the cathode 20 are formed into an electron beam by the control grid 28 and the screen grid 30. The cathode 20 is connected to a suitable voltage source 34 which may be at ground potential. A source 36 of video signals may be connected to the cathode 20 by a switch 38 in some applications of my invention where the image written onto the target member 14 is done by an electron beam rather than a light directed onto the target member 14.

The control grid 28 is supplied with a suitable negative potential of about 45 to 95 volts from a source 40. It may be desirable in some applications to'modulate the control grid 28 by connecting the video source 36 to the control grid rather than the cathode. The technique of modulating the electron beam for writing is well-known in the art. The screen or accelerating grid 30 is supplied with -a positive potential of about 300 volts from a source 42. The beam focus electrode 32 is supplied with a positive potential of about 250 volts from a source 44. A fine mesh 56 is mounted adjacent to the target 14 and is electrically connected to the electrode 32 and operates at the same potential.

The electrons generated by the electron gun 17 are magnetically focused to a small area spot at the target 14 by a magnetic field provided by the focus coil 50. An alignment coil 52 may also be provided to correct for misalignment of the electron gun 17. A deflection yoke 54 is provided for scanning the electron beam over the surface of the target 14. During operation, suitable voltages are applied to the system 54 to provide the necessary scan.

In the conventional vidicon, the electron beam may scan the surface of the storage target either at a high or low velocity. In the more conventional low velocity type of operation, the electrons approach at a velocity below the first crossover potential of the film of material on the target member. Electrons are deposited on the target and would tend to charge the surface in a negative direction toward an equilibrium potential which approaches the cathode potential of the gun which is found in the specific embodiment described herein. It operated in the high velocity mode, the electrons strike the surface of the target between first and second crossover potential of the material and the target surface will tend to be charged in a positive direction by the electron beam. In the case of the low velocity mode operation as described herein, the conductive back plate is connected through a resistance to establish a potential of about 10 volts .positive on the signal plate with respect to the equilibrium potential.

Referring now to FIGURE 2, for a more detailed description of the target 14, the target 14 is supported on the light transmissive face plate 12. The target 14 consists of an electrically conductive coating 16 of a thickness of about 500 angstroms and transmissive to the input radiation. The resistance of the material in the layer 16 should be less than 200 ohms per square. The electrically conductive coating 16 may be of a suitable material such as tin oxide and is formed by spraying a solution of tin salt over the heated support face plate 12. It is also possible to evaporate an electrically conductive coating such as gold according to well-known and established techniques onto the face plate 12 to provide the conductive coating.

After the electrically conductive coating has been provided upon the face plate, the structure is placed in a vacuum of approximately 10* millimeters of mercury. A boat of suitable material such as tantalum is charged with about 600 milligrams of CdAs Se S and set at a throw distance of about six inches. The material CdAs Se S may be prepared by adding 1.1226 grams of cadmium, 1.4962 grams of arsenic, 2.3656 grams of selenium and 0.3203 gram of sulfur. This mixture is placed in a small quartz bulb and sealed off under a vacuum of 10- millimeters of mercury or better and then placed in a furnace. I

The bulb is heated to a temperature of about 900 C. for a period of one-half hour and the contents are mixed during this heating operation by shaking, for example. The bulb is then removed from the furnace and cooled rapidly to room temperature. The process provides a suitable composition of CdAs Se S for evaporation onto the target. The boat is then heated to a temperature of 700 C. and the material evaporated onto conductive coating. The target as thus prepared is then positioned in the evacuated envelope and the face plate is sealed to the envelope and the envelope evacuated.

It is believed that the two elements arsenic and selenium are the necessary components in this storage structure and that the presence of anyone of the elements cadmium, mercury, silver and zinc in intimate admixture provide a vary from As Superior storage electrode layer. Examples of suitable formulations are: ZnAs Se HgAs Se AgAsSe CdAS2Se4 and CdAS2SE3S.

In the specific examples given above, the materials evaporated are in stoichiometric composition. It has been found that deviations in the composition from these stoichiometric proportions will still give beneficial results. Generally up to a 20% variation of any element in the above examples is suitable. Thus, in the above formula, Cd As Se S cadmium could range from Cd to Cd arsenic could to AS selenium could vary fron Se to Sc and sulfur could vary from S m 1.2- proportions indicated above would be CdAs Se S Cd As Se S, and OdAs Se S It is apparent that one or more of the elements may vary independently. Each of these formulas provide slight variations in the storage properties of the target.

In addition to the above modifications of the stoichiometric composition by up to 20 atomic percent of the elements, it is also possible to modify the composition by substitutions as follows. For example, up to 2.0 atomic percent of certain other elements may be substituted for cadmium, namely zinc or mercury or silver up to 20 atomic percent of phosphorus or antimony or both may be substituted for arsenic, and up to 35 atomic percent of tellurium may be substituted for either the selenium or sul fur. Thus on a formula basis the following materials These storage compositions may be produced by a prolonged ball milling of the respective elements to provide an initimate admixture. By the term an intimate admixture is meant a mechanical admixture as produced by ball milling, a compound or a solid solution.

In the operation of the device, the voltages previously mentioned are applied to the electrodes and a light image is directed onto the target by suitable focusing means and then the face plate capped to prevent any further illumination of the target. It is then found that the light image provides a charge image on the target 14 which does not extinguish as the scan beam moves over the target and the image maybe retained on the target and scanned for a period of longer than one hour with conventional television scanning techniques wherein 30 frames per second are scanned. It is found that the low velocity reading beam does not erase the charge as in a conventional vidicon, but instead has the property of retaining the charge image on the target member. The properties of the target are clearly illustrated in FIGURE 3 wherein the curve 80 represents the integration properties of the target and curve 82 indicates the amount of signal derived from the target over a period of time of more than one hour. The dotted lines 8-1 and 83 are corresponding curves for prior art devices. Even at the end of the one hour of scan, storage images are still obtained from the. storage tube and when displayed on a conventional television monitor include more than adequate half tone properties.

In FIGURE 4, the properties of the target member are illustrated, with curve '84 illustrating a structure in which the signal plate voltage is 16 volts, curve 86 in which the signal plate voltage is 14 volts and curve 88 in which the signal plate is volts. FIGURE 5 simply illustrates the integrated response of the target according to the wave length of the light impressed on the light sensitive material.

To erase the stored signal on the target, it is necessary to bias the reading beam to cut-off or reduce the target voltage toprevent beam landing on the target. Failure of the beam to land results in the image decaying to substantially zero in a time period of about one minute If it is desired that the information on the target be erased almost instantaneously, then a uniform high level source of illumination may be directed onto the target.

Examples of suitable formulation varying in the Although We have described this invention in only one specific embodiment in which a light sensitive target has been described, the target member has a wider variety of tube applications as indicated previously in an electron bombardment induced conductivity type of target wherein the writing information is directed onto the target by means as a scanning electron beam positioned on one side of the target .and the information readout from the other side of the target in the conventional manner described above. It has also been previously mentioned that it is possible to control the persistence of the image by adjusting the target voltage. It should also be noted here that an X-ray sensitive phosphor may be utilized in the case of where .an X-ray input is directed onto the pickup tube. This would permit conversion of the X-rays into a light image to which the photoconductive layer would be sensitive.

While there have been shown and described what are presently considered to be the preferred embodiments of the invention, modifications thereof will readily occur to those skilled in the art. It is not desired, therefore, that the invention be limited to the specific arrangements shown and described, and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

We claim as our invention:

1. A storage tube comprising a storage electrode, said storage electrode including a layer derived by evaporating a material comprised of substantial amounts-of each of arsenic and selenium and including in addition an effective amount of at least one element selected from the group consisting of zinc, silver, cadmium and mercury, said layer exhibiting the property of storing an information pattern in response to electron bombardment, including halftones, in response to excitation and discharge of said information pattern in a predetermined time in the absence of electron bombardment, said layer also exhibiting the property of retaining said stored pattern for a greater length of time than said predetermined time in response to a constant current of electrons, means for directing excitation onto said storage electrode to establish a storage conductivity pattern in said storage electrode and means for directing an electron beam onto said target to retain said stored pattern for a greater length of time than said predetermined time.

2. The storage electrode of claim 1 wherein a substantial amount of sulfur is present in intimate admixture with said arsenic and selenium.

3. The storage electrode of claim 1 wherein up to 35 atomic percent of sulfur is substituted for the selenium.

4. The storage electrode of claim 1 in which the atomic ratio of arsenic to selenium is within the atomic ratio of from 1:1 to 1:3.

5. A storage tube comprising a storage electrode, said storage electrode including a layer comprising essentially an intimate admixture of substantial amounts of arsenic, selenium and sulfur and including in addition an effective amount of at least one element selected from the group consisting of zinc, silver, cadmium and mercury, said layer exhibiting the property of storing an information pat-tern, including halftones in response to excitation and discharge of said information pattern in a predetermined time in the absence of electron bombardment, said layer also exhibiting the property of retaining said stored pattern for a greater length of time than said predetermined time in response to a constant current of electrons, means for directing excitation onto said storage electrode to establish a storage conductivity pattern in said storage electrode and means for directing an electron :beam onto said target to retain said stored pattern for a greater length of time than said predetermined time.

6. The storage electrode of claim 5 wherein tellurium is substituted for said sulfur up to 30 atomic percent.

7. The storage tube comprising a storage electrode, said storage electrode including a layer derived by evaporating a material composed of CdAs Se S wherein there is provided up to a 20 percent variation of any element from the stoichiometric composition for storing an information pattern including halftones in response to excitation and discharge of said pattern in a predetermined time in the absence of electron bombardment, said layer also for retaining said stored pattern for a greater length of time than said predetermined time in response to a constant current of electrons, means for directing excitation onto said target to establish a storage conductivity pattern in said target and means for directing electron beam onto said target to retain said stored pattern for a greater length of time than said predetermined time.

8. The storage electrode of claim 7 wherein up to 20 atomic percent of the cadmium is substituted by at least one element of the group consisting of zinc, mercury and silver.

9. The storage electrode of claim 7 wherein up to 20 atomic percent of the arsenic is substituted for by at least one element of the group consisting of phosphorus and antimony.

10. The storage electrode of claim 7 wherein up to 20 atomic percent of the selenium is substituted for by tellurium.

11. The storage electrode of claim 7 in which up to 20 atomic percent of the sulfur is substituted for by tellurium.

References Cited by the Examiner UNITED STATES PATENTS GEORGE N. WESTBY, Primary Examiner.

R. SEGAL, Assistant Examiner. 

1. A STORAGE TUBE COMPRISING A STORAGE ELECTRODE, SAID STORAGE ELECTRODE INCLUDING A LAYER DERIVED BY EVAPORATING A MATERIAL COMPRISED OF SUBSTANTIAL AMOUNTS OF EACH OF ARESNIC AND SELENIUM AND INCLUDING IN ADDITION AN EFFECTIVE AMOUNT OF AT LEAST ONE ELEMENT SELECTED FROM THE GROUP CONSISTING OF ZINC, SILVER, CADMIUM AND MERCURY, SAID LAYER EXHIBITING THE PROPERTY OF STORING AN INFORMATION PATTERN IN RESPONSE TO ELECTRON BOMBARDMENT, INCLUDING HALFTONES, IN RESPONSE TO EXCIATION AND DISCHARGE OF SAID INFORMATION PATTERN IN A PREDETERMINED TIME IN THE ABSENCE OF ELECTRON BOMBARDMENT, SAID LAYER ALSO EXHIBITING THE PROPERTY OF RETAINING SAID STORED PATTERN FOR A GREATER 